This article, originally a lecture given at Brigham Young University in 2009, was published as part of a special issue of BYU Studies featuring Thomas L. Kane. Although Kane was not a member of The Church of Jesus Christ of Latter-day Saints, he was an advocate for the Mormon cause and a trusted friend of Mormon leaders for almost forty years. This article focuses on the correspondence between Kane and the Mormon prophet Brigham Young. There are about 125 known letters exchanged between Young and Kane, beginning the year they met in 1846 and extending to 1877, the year Young died. The number of letters averaged three or four per year, with more frequent exchanges during times of crisis. The Brigham Young–Thomas L. Kane letters are an important source for understanding both men, as well as various aspects of Latter-day Saint and American history. The letters also provide a window into one of those rare, enduring friendships that help reveal the times in which the writers lived.